1. Field of the Invention
The invention relates to a method of converting a digital colour image to a digital colour image with improved colour distinction, and more particularly to a method of this kind suitable for improving colour distinction in a colour image for an observer suffering from a type of colour-blindness.
2. Discussion of the Related Art
People suffering from colour-blindness may encounter difficulties with certain documents containing colours. Pie charts, histograms, maps, etc. can be unreadable for these people. A good design of software or web pages should consider the fact that colour-blind computer users see things differently than most users, as taught by an article entitled “The Eye of the Beholder—Designing for Colour-Blind Users” by C. Rigden in British Telecommunications Engineering, Vol. 17, 443-451 (1999). A method described therein consists in representing images as perceived by people with a certain colour deficiency and using the representation as a check after the conception of the document. In order to perform the check, use is made of transformed colour palettes in which each colour of a web-safe palette has been transformed to a colour value as perceived by a colour-blind person. That way, designers can produce documents, software interfaces and web pages which avoid problems for their users. However, this kind of adapted design is seldom applied in reality. Moreover, it has the disadvantage that the design is optimized for the most common deficiency with red and green, while other deficiencies receive less attention.
When colours in a document are used as primary means of information, this can become an issue for colour-blind people. Changing the colours of such existing documents in a given way may enable the colour-blind people to distinguish some colours which would be otherwise perceived mainly equally by them. After such transformation, these documents may be printed or displayed on a screen, by which these people can retrieve the information contained in the original colours.
A method of correcting a digital image for colour-blinds is known from Vischeck (www.vischeck.com/daltonize/). In this method, an image processing algorithm is used to improve the situation of people with a colour deficiency. The Vischeck algorithm is a combination of the following steps:                the red/green contrast in the image is increased;        the information conveyed by variations in the red/green direction is analyzed and converted into changes in brightness; and        the information conveyed by variations in the red/green direction is analyzed and converted into changes in blue/yellow coloration.        
One step of the Vischeck algorithm makes the red/green variations somewhat more visible to colour-blinds, since many of these people posses some residual red/green discrimination. Another step according to this algorithm performs a mapping of some information contained in the document from an invisible colour dimension for colour-blinds into a colour dimension that they can see.
The Vischeck algorithm, however, has the inconvenience that the type of correction has to be given by hand by the user, meaning that the user has to give for each of the steps of the algorithm a numerical value. This makes the use of the algorithm cumbersome for a user who does not know which values apply to him. Even after a lot of trials, the improvement is still not convincing due to a lack of performance of the algorithm. There are also situations in which this algorithm does not work, meaning that the colour-blind user still may not be able to retrieve correctly the information contained in the document.